String _authToken;

Map<String, String> _getRequestHeaders() {
    return {
      'Authorization': 'Bearer $_authToken'
    };
  }

Observable<User> _login(String username, String password) {
    return Observable.fromFuture(_get('/login?username=$username&password=$password'))
        .map((res) {
          _authToken = res.body.token;
          return User(res.body.data);  /** Using manual Serialization in User model **/
    });
  }